Vim Spelling Underline Shows in urxvt but not xfce-terminal

Question

When I enable vim's spellchecker using :set spell underlines appear under any misspelled words in urxvt.

However, when I try the same in xfce-terminal (on the same machine, using the same .vimrc and ediing the same file) no underlines appear. The spellchecker is still working as misspelled words can be navigated between, it is just the underlines that are not showing.

Does anyone know what settings in vim or xfce-terminal are causing this to happen.

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Edit: This seems to be an issue specifically with vim rather than the terminal itself. Underlines printed using echo still display correctly, i.e:

echo -e "\e[4munderline\e[0m"

Answer 1

This problem is due to vim trying to use undercurls (wiggly underlines) rather than normal underlines, but being configured incorrectly.

Check that your terminal supports undercurls by trying to print some:

echo -e '\e[4:3mcurly underline (new in 0.52)\e[4:0m'

If they display correctly then the problem is due to vim. Most likely due to vim's t_Cs and t_Ce variable being set incorrectly.

There is a related issue here, which recommends adding the following to your .vimrc:

let &t_Cs = "\e[4:3m"
let &t_Ce = "\e[4:0m"

After doing this undercurls started being displayed in vim for me.